Primary (non-rechargeable) electrochemical cells having an anode of lithium are known and are in widespread commercial use. The anode is comprised essentially of lithium metal. Such cells typically have a cathode comprising manganese dioxide, and electrolyte comprising a lithium salt such as lithium trifluoromethane sulfonate (LiCF3SO3) dissolved in a nonaqueous solvent. The cells are referenced in the art as primary lithium cells (primary Li/MnO2 cells) and are generally not intended to be rechargeable. Alternative primary lithium cells with lithium metal anodes but having different cathodes, are also known. Such cells, for example, have cathodes comprising iron disulfide (FeS2) and are designated Li/FeS2 cells. The iron disulfide (FeS2) is also known as pyrite. The Li/MnO2 cells or Li/FeS2 cells are typically in the form of cylindrical cells, typically AA size or AAA size cells, but may be in other size cylindrical cells. The Li/MnO2 cells have a voltage of about 3.0 volts which is twice that of conventional Zn/MnO2 alkaline cells and also have higher energy density (watt-hrs per cm3 of cell volume) than that of alkaline cells. The Li/FeS2 cells have a voltage (fresh) of between about 1.2 and 1.8 volts which is about the same as a conventional Zn/MnO2 alkaline cell. However, the energy density (watt-hrs per cm3 of cell volume) of the Li/FeS2 cell is higher than a comparable size Zn/MnO2 alkaline cell. The theoretical specific capacity of lithium metal is high at 3861.4 mAmp-hr/gram and the theoretical specific capacity of FeS2 is 893.6 mAmp-hr/gram. The FeS2 theoretical capacity is based on a 4 electron transfer from 4Li per FeS2 molecule to result in reaction product of elemental iron Fe and 2Li2S. That is, 2 of the 4 electrons change the oxidation state of +2 for Fe+2 in FeS2 to 0 in elemental iron (Fe0) and the remaining 2 electrons change the oxidation state of sulfur from −1 in FeS2 to −2 in Li2S. In order to carry out the electrochemical reaction the lithium ions, Li+, produced at the anode must transport through the separator and electrolyte medium and to the cathode.
Overall the Li/FeS2 cell is much more powerful than the same size Zn/MnO2 alkaline cell. That is for a given continuous current drain, particularly at higher current drain over 200 milliAmp, the voltage is flatter for longer periods for the Li/FeS2 cell than the Zn/MnO2 alkaline cell as may be evident in a voltage vs. time profile. This results in a higher energy output obtainable from a Li/FeS2 cell compared to that obtainable for a same size alkaline cell. The higher energy output of the Li/FeS2 cell is also clearly shown more directly in graphical plots of energy (Watt-hrs) versus continuous discharge at constant power (Watts) wherein fresh cells are discharged to completion at fixed continuous power outputs ranging from as little as 0.01 Watt to 5 Watt. In such tests the power drain is maintained at a constant continuous power output selected between 0.01 Watt and 5 Watt. (As the cell's voltage drops during discharge the load resistance is gradually decreased raising the current drain to maintain a fixed constant power output.) The graphical plot Energy (Watt-Hrs) versus Power Output (Watt) for the Li/FeS2 cell is above that for the same size alkaline cell. This is despite that the starting voltage of both cells (fresh) is about the same, namely, between about 1.2 and 1.8 volt.
Thus, the Li/FeS2 cell has the advantage over same size alkaline cells, for example, AAA, AA, C or D size or any other size cell in that the Li/FeS2 cell may be used interchangeably with the conventional Zn/MnO2 alkaline cell and will have greater service life, particularly for higher power demands. Similarly the Li/FeS2 cell which is a primary (nonrechargeable) cell can be used as a replacement for the same size rechargeable nickel metal hydride cell, which has about the same voltage (fresh) as the Li/FeS2 cell.
The Li/MnO2 cell and Li/FeS2 cell both conventionally employ nonaqueous electrolytes, since the lithium anode is reactive with water. The electrolyte for the Li/MnO2 or Li/FeS2 cell typically comprises specific electrolyte salts such as LiCF3SO3 (LiTFS) or Li(CF3SO2)2N (LiTFSI) dissolved in specific combinations of organic solvents. Generally, it has been the practice not to add water to the electrolyte solvents and to specify to the supplier that the water content in such solvents be limited to trace amount. That is, it has been the practice that any water present in the electrolyte solvent for the primary Li/MnO2 cell preferably be less than about 50 parts by weight water per one million parts by weight solvent. (See, e.g. U.S. Pat. No. 6,280,883 B1 describing water content in electrolyte solvent for many lithium cells at less than about 100 ppm, preferably less than 50 ppm. (However, the Li/FeS2 cell is not specifically mentioned in this reference.) In any event it has been the practice not to add water to the electrolyte solvent for lithium cells. The supplier may typically subject the electrolyte solvent to drying or purification to assure that the water content in the solvent is well within the purchaser's specification. Such restriction of water content in the electrolyte solvent has been applied in particular to commercial primary Li/MnO2 cells. Since it was considered standard practice to keep water content as low as possible in the Li/MnO2 cell, it is natural to extend this practice to more recent primary lithium cells, in particular the Li/FeS2 cell. With respect to the Li/FeS2 primary cell, the reference US 2005/0277023 A1 teaches that water is an electrolyte contaminant and there should be no more than 500 ppm by weight water in the electrolyte. (para. 122, lines 1-5)
Certain aqueous electrolyte systems, that is, electrolyte systems with added water therein, may have higher electrical conductivity than organic non aqueous electrolyte systems. This is because water may typically promote better ionization of the electrolyte salt than an organic solvent. However in lithium cells, such as the Li/MnO2, it has been considered important to restrict water content in the electrolyte solvent to trace amount and not to add water, primarily because water can react with lithium metal and also with electrolyte material including electrolyte solvent. This can produce a reaction product which coats the surface of the lithium anode. Such coating may be termed a “passivation layer” on the lithium anode surface which has the potential of significantly reducing the discharge performance and efficiency of the cell. However, not all “passivation layers” on the lithium anode are the same. That is, the chemical composition of such passivation layer and its rate of buildup (stability) on the surface of the anode may differ depending on the amount of water added to the electrolyte and the specific electrolyte salt and electrolyte solvents which are employed.
Thus, Applicants herein have determined that depending on the chemical composition of the electrolyte, which includes specific electrolyte salt and solvents employed, and the amount of added water, it is possible to produce a passivation layer on the lithium anode for the Li/FeS2 cell which does not significantly compromise cell performance. At the same time, because water is added to the electrolyte, the electrolyte is more conductive, thus promoting very good cell discharge performance overall for the Li/FeS2 cell.
The cathode material may be initially prepared in the form of a slurry mixture, which can be readily coated onto a substrate, typically a metal substrate, by conventional coating methods. The electrolyte added to the cell is a suitable electrolyte for the Li/FeS2 system allowing the necessary electrochemical reactions to occur efficiently over the range of high power output desired. The electrolyte must exhibit good ionic conductivity and also be sufficiently stable, that is non reactive, with the undischarged or partially discharged electrode materials (anode and cathode components) and also non reactive with the discharge products. This is because undesirable oxidation/reduction reactions between the electrolyte and electrode materials (either discharged or undischarged or partially discharged) could gradually contaminate the electrolyte and reduce its effectiveness or result in excessive gassing. This in turn can result in a cell failure. Thus, the electrolyte used in a Li/FeS2 cell in addition to promoting the necessary electrochemical reactions, should also be stable in contact with discharged, partially discharged and undischarged electrode materials. The electrolyte solvent should enable good ionic mobility and transport of the lithium ion (Li+) from anode to cathode so that it can engage in the necessary reduction reaction resulting in Li2S product in the cathode.
Primary lithium cells are in use as a power source for digital flash cameras, which require operation at higher pulsed power demands than is supplied by individual alkaline cells. Primary lithium cells are conventionally formed of an electrode composite comprising an anode formed of a sheet of lithium (or lithium alloy, essentially of lithium), a cathode formed of a coating of cathode active material comprising FeS2 on a conductive metal substrate (cathode substrate) and a sheet of electrolyte permeable separator material therebetween. A microporous polypropylene separator for a lithium cell is disclosed, for example, in U.S. Pat. No. 4,794,057. The electrode composite may be spirally wound and inserted into the cell casing, for examples, as shown in U.S. Pat. No. 4,707,421.
A portion of the spiral wound anode sheet is typically electrically connected to the cell casing which forms the cell's negative terminal. The cell is closed with an end cap which is insulated from the casing. The cathode sheet can be electrically connected to the end cap which forms the cell's positive terminal. The casing is typically crimped over the peripheral edge of the end cap to seal the casing's open end. The cell may be fitted internally with a PTC (positive thermal coefficient) device or the like to shut down the cell in case the cell is exposed to abusive conditions such as short circuit discharge or overheating.
The anode in a Li/FeS2 cell can be formed by laminating a layer of lithium metal or lithium alloy on a metallic substrate such as copper. However, the anode may be formed of a sheet of lithium or lithium alloy without any substrate.
The electrolyte used in primary Li/FeS2 cells is formed of a “lithium salt” dissolved in an “organic solvent”. The electrolyte must promote ionization of the lithium salt and provide for good ionic mobility of the lithium ions so that the lithium ions may pass at good transport rate from anode to cathode through the separator. Representative lithium salts which may be used in electrolytes for Li/FeS2 primary cells are referenced in U.S. Pat. No. 5,290,414 and U.S. Pat. No. 6,849,360 B2 and include such salts as: Lithium trifluoromethanesulfonate, LiCF3SO3 (LiTFS); lithium bistrifluoromethylsulfonyl imide, Li(CF3SO2)2N (LiTFSI); lithium iodide, LiI; lithium bromide, LiBr; lithium tetrafluoroborate, LiBF4; lithium hexafluorophosphate, LiPF6; lithium hexafluoroarsenate, LiAsF6; lithium methide, Li(CF3SO2)3C; LiClO4; lithium bis(oxalato)borate, LiBOB and various mixtures. In the art of Li/FeS2 electrochemistry lithium salts are not always interchangeable as specific salts work best with specific electrolyte solvent mixtures.
In U.S. Pat. No. 5,290,414 (Marple) is reported use of a beneficial electrolyte for FeS2 cells, wherein the electrolyte comprises a lithium salt dissolved in a solvent comprising 1,3-dioxolane (DX) in admixture with a second solvent which is an acyclic (non cyclic) ether based solvent. The acyclic (non cyclic) ether based solvent as referenced may be dimethoxyethane (DME), ethyl glyme, diglyme and triglyme, with the preferred being 1,2-dimethoxyethane (DME). As given in the example the 1,2-dimethoxyethane (DME) is present in the electrolyte in substantial amount, i.e., at either 40 or 75 vol. % (col. 7, lines 47-54). A specific lithium salt ionizable in such solvent mixture(s), as given in the example, is lithium trifluoromethane sulfonate, LiCF3SO3. Another lithium salt, namely lithium bistrifluoromethylsulfonyl imide, Li(CF3SO2)2N is also mentioned at col. 7, line 18-19. The reference teaches that a third solvent may optionally be added selected from 3,5-dimethylisoxazole (DMI), 3-methyl-2-oxazolidone, propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), tetrahydrofuran (THF), diethyl carbonate (DEC), ethylene glycol sulfite (EGS), dioxane, dimethyl sulfate (DMS), and sulfolane (claim 19) with the preferred being 3,5-dimethylisoxazole.
In U.S. Pat. No. 6,849,360 B2 (Marple) is disclosed a specific preferred electrolyte for an Li/FeS2 cell, wherein the electrolyte comprises the salt lithium iodide dissolved in the organic solvent mixture comprising 1,3-dioxolane (DX), 1,2-dimethoxyethane (DME), and small amount of 3,5 dimethylisoxazole (DMI). (col. 6, lines 44-48) The electrolyte is typically added to the cell after the dry anode/cathode spiral with separator therebetween is inserted into the cell casing.
In US 2007/0202409 A1 (Yamakawa) it is stated with reference to the electrolyte solvent for the Li/FeS2 cell at para. 33: “Examples of the organic solvent include propylene carbonate, ethylene carbonate, 1,2-dimethoxy ethane, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, sulfolane, acetonitrile, dimethyl carbonate, and dipropyl carbonate, and any one of them or two or more of them can be used independently, or in a form of a mixed solvent.” Such statement is misleading, since the art teaches only specific combinations of electrolyte solvents will be workable for the Li/FeS2 cell depending on the particular lithium salt to be dissolved in the solvent. (See, e.g. above U.S. Pat. Nos. 5,290,414 and 6,849,360) The reference Yamakawa does not teach which combination of solvents from the above list are to be used with any given lithium salt.
Conventional FeS2 powders, for example Pyrox Red 325 powder from Chemetall GmbH, are conventionally available with pH raising additives therein to offset or retard any buildup in acidity of the powder. Such additives are believed to contain calcium carbonate (CaCO3) or calcium carbonate linked to other compounds. Such calcium carbonate is added to the FeS2 powder to retard the formation of acidic impurities within or on the surface of the powder as it is stored in ambient air and exposed to oxygen and moisture. This is regardless of whether the FeS2 is intended for use in cathode mixtures or other applications, for example, as an additive in manufacture of car brakes.
Accordingly, it is desired to realize the benefit of improving the electrical conductivity of one or more organic electrolyte systems for the Li/FeS2 cell by adding water to the electrolyte.
It is desired to find a range of amount water that can be added to the Li/FeS2 electrolyte in order to realize beneficial cell performance resulting from improved electrolyte conductivity, while containing deleterious side reactions from reaction of water with cell components.
It is desired to produce a primary (nonrechargeable) Li/FeS2 cell having good rate capability that the cell may be used in place of rechargeable batteries to power digital cameras.